When the Presario will not boot, you need to take the necessary steps to get it back in service. A number of things can cause a Presario to fail to boot, so it is best to take a step-by-step approach to troubleshooting and resolving the issue.
Shut the Presario computer down and remove the power cord from the back. Remove the retaining screws from the computer case.
Locate the case removal lever and pull it up to remove the case cover. For desktop computers the lever should be on the top. For tower computers the lever will be on the side. Set the case cover aside.
Find the CMOS button or jumper on the system board. Many Presario models use a blue button to control the CMOS settings on the computer, while others use a small jumper. To reset the CMOS settings and clear any problems that are preventing the computer from starting, press the blue button for 10 seconds, or pull the white plastic piece off the jumper and leave it off for 10 seconds.
Press the "Power" button on the Presario and watch the fan on the power supply. The fan and the power supply are located on the left-hand side at the back of the computer. If the fan runs slowly, it might be blocked by dust that has been sucked into the computer. If the fan does not run at all the power supply is probably bad.
Spray the interior of the computer with a can of compressed air to dislodge any debris. If the fan still does not spin, the power supply will need to be replaced.